The invention is directed to a flow sensing device and a method for sensing flow.
Flow sensing devices use physical principles to produce an analog output to measure flow rates. The analog output can also be integrated to calculate flow volumes. Flow sensing devices include a pressure differential type, which has an air-resistive element that creates a pressure drop, which is proportional to the flow of fluid or air through the tube in which the resistive element is located. A pressure transducer converts the pressure reading into electrical signals, which can be used to determine the air flow rate, which can be integrated to provide volume measurements.
Flow sensing devices can be used for measuring air flow in a respiratory air flow apparatus. Such apparatus can include apparatus for artificial ventilation of a patient, pneumotachometers, equipment for assessing cardiopulmonary performance and evaluating pulmonary function during exercise and static testing, air breathing apparatus such as emergency apparatus and scuba respirators and the like.
The present invention is directed to a pneumotachometer which is a device that measures instantaneous respiratory air flow. In a differential pressure flow transducer pneumotachometer, a sensitive manometer detects a pressure drop across a light resistance placed in the air flow. For example, a Fleisch pneumotachometer utilizes capillary air flow resulting from air flowing through a resistant element made of a bundle of parallel capillary tubes to maintain a linear relationship between flow and pressure difference. The tubes can be layers of bundles of fine metal screen capillaries that cause a linear resistance to lateral flow of air through the tube.
One type of conventional respiratory air sensing device 10 is shown in FIG. 1 from Tillotson et al., U.S. Pat. No. 5,676,132. The device 10 utilizes a venturi-type tube 12. A venturi is a constriction section of a pipe that causes a drop in pressure as fluid flows through the pipe. The venturi 12 includes a short straight or through pipe section 14 between two tapered sections 16, 18 and can be used to measure fluid flow rate through the pipe. The Tillotson et al. device includes a midsection area 14 of lesser diameter that offers a resistance to flow through the tube. In a respiratory device, the constricted midsection area 14 causes the flow of air that is expired 20 or inspired 22 by a subject, user or patient at a first end 24 of the venturi tube 12 to become a laminar stable air flow in the midsection area 14. A sensor 26 such as a microsensor, is arranged in the midsection area 14 with pins 28 extending outside of the venturi tube 12. The sensor 26 protrudes from an inner surface of the tube 12 into the laminar air flow to detect the flow rate of air therethrough. The sensor 26 then sends detections signals to a microprocessor (not shown) via the pins 28 and a connector 30.
Flow through the venturi tube can be represented by a Bernoulli model where as the cross section of the tube is restricted, flow velocity increases. So long as flow through a tube is laminar (non turbulent), change in pressure created by restricting walls of the tube is described by the equation xcex94P=axcex7V+bxcex3V2 where V is gas flow, xcex7 is dynamic viscosity, xcex3 is gas density and a and b are constants determined by the flow tube characteristics and type of restriction. In the case of the venturi, and assuming an isothermic system and an incompressible fluid, pressure drop created by the cross sectional area of the restriction is represented by xcex94P=Cxcex3V2 and change in pressure in the tube is represented by xcex94P=axcex7V+bxcex3V2xe2x88x92Cxcex3V2. The relationship can be reduced to a simple linear relationship, xcex94P=axcex7V.
A problem with venturi-type air sensing devices is the inability to accurately sense flow across a full range of flow rates. For example, at low flow rates, the Reynolds number of the fluid exceeds a critical level and the fluid flow becomes turbulent and non uniform. In turbulent flow, local velocities and pressures of fluid fluctuate irregularly and in a random manner. This results in a non-linear relationship between fluid flow and pressure. Further at low flow rates, the converging shape of the venturi section may reflect back flow to give incorrect readings.
The present invention is directed to a combined pressure change sensing mechanism that provides accurate measurement of pressure difference of turbulent flow as well as of linear flow. In another aspect, the present invention is directed to a disposable fluid sensing device that is easy to manufacture and that includes a safety feature to prevent multiple use.
The fluid sensing device of the invention comprises a venturi tube and a fluid resistive element. The venturi tube defines a first open end through which a flow of fluid enters, a second open end from which fluid flow exits and a constricted midportion therebetween. The fluid resistive element is located within the venturi tube. In combination with the tube, the fluid resistive element measures flow for accurate sensing of the flow over a range of fluid flow into the tube.
In another aspect, the invention relates to a labeled fluid sensing device, comprising a fluid flow sensing tube and a label. The label comprises a first panel secured to a surface of the tube and a second optically sensable panel removably connected to the first panel.
In another aspect, the invention relates to a process of evaluating fluid flow. In the process a venturi tube and a fluid resistive element located within the tube are provided. The combination of the resistive element with the venturi tube linearizes flow for accurate sensing of said flow over a range of fluid flow into the tube. Fluid is caused to flow into the venturi tube through the first end to exit the second. The flowing fluid within the tube is sensed to provide flow data according to the fluid flow.
In still another aspect, the invention relates to a respiratory fluid flow monitoring process, wherein a venturi tube, a flow sensor and a flow resistive element are provided. The venturi tube includes a first open end through which a flow of fluid enters, a second open end from which fluid flow exits and a constricted midportion therebetween. The flow sensor is positioned within the tube midportion to provide flow data on the fluid flowing through the venturi tube. The fluid resistive element is located within the venturi tube at a location between the first open end and the flow sensor to linearize flow of fluid across a range of flow rates. Fluid is caused to flow into the venturi through the first end to exit the second end. The flowing fluid is sensed with the flow sensor to provide flow data and a flow of the fluid is adjusted according to the flow data.
In a final aspect of the invention, a fluid flow process comprises inserting a tube into a fluid flow system, monitoring the insertion of the tube with an optical sensor to determine absence or presence of an optically sensed indicia on the tube and terminating the process if the monitoring determines that the optically sensed indicia is absent.